Experimental and Numerical Investigation of Natural Convection Within Vertical Annulus

Abstract

Natural convection flow in a vertical annulus with an isothermally heated bottom and a uniformly cooled top surface is experimentally investigated using laser visualization technique. The outer and inner walls of the annulus are adiabatic. The annulus is heated from the bottom, thereby causing upward movement of air which is then cooled by the top wall. This heat transfer process leads to the formation of natural convection loops in the annulus. These convection loops were visualized using laser visualization techniques. Three dimensional, unsteady and a highly unstable flow was observed for a Rayleigh number 2.86 × 10⁶ within the annulus having height 650 mm, aspect ratio 10 and annular gap 70 mm. To validate the experimental results, an in-house numerical code was developed in FORTRAN. The numerical results also show the existence of three dimensional, unsteady and chaotic flows in the annulus. A correlation for the average Nusselt number for varying heat inputs to the bottom plate and closed top plate was developed using SPSS 16 software. The Nusselt number obtained from the correlation was plotted against experimental values and showed an almost linear variation within specified error limits.